/*
 * Copyright (C) 2010 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.RandomAccess;
import javax.annotation.Nullable;

/**
 * Static methods pertaining to sorted {@link List} instances.
 *
 * In this documentation, the terms <i>greatest</i>, <i>greater</i>, <i>least</i>, and <i>lesser</i>
 * are considered to refer to the comparator on the elements, and the terms <i>first</i> and
 * <i>last</i> are considered to refer to the elements' ordering in a list.
 *
 * @author Louis Wasserman
 */
@GwtCompatible
@Beta
final class SortedLists {
    private SortedLists() {}

    /**
     * A specification for which index to return if the list contains at least one element that
     * compares as equal to the key.
     */
    public enum KeyPresentBehavior {
        /**
         * Return the index of any list element that compares as equal to the key. No guarantees are
         * made as to which index is returned, if more than one element compares as equal to the
         * key.
         */
        ANY_PRESENT {
            @Override
            <E> int resultIndex(Comparator<? super E> comparator, E key, List<? extends E> list, int foundIndex) {
                return foundIndex;
            }
        },
        /**
         * Return the index of the last list element that compares as equal to the key.
         */
        LAST_PRESENT {
            @Override
            <E> int resultIndex(Comparator<? super E> comparator, E key, List<? extends E> list, int foundIndex) {
                // Of course, we have to use binary search to find the precise
                // breakpoint...
                int lower = foundIndex;
                int upper = list.size() - 1;
                // Everything between lower and upper inclusive compares at >= 0.
                while (lower < upper) {
                    int middle = (lower + upper + 1) >>> 1;
                    int c = comparator.compare(list.get(middle), key);
                    if (c > 0) {
                        upper = middle - 1;
                    } else { // c == 0
                        lower = middle;
                    }
                }
                return lower;
            }
        },
        /**
         * Return the index of the first list element that compares as equal to the key.
         */
        FIRST_PRESENT {
            @Override
            <E> int resultIndex(Comparator<? super E> comparator, E key, List<? extends E> list, int foundIndex) {
                // Of course, we have to use binary search to find the precise
                // breakpoint...
                int lower = 0;
                int upper = foundIndex;
                // Of course, we have to use binary search to find the precise breakpoint...
                // Everything between lower and upper inclusive compares at <= 0.
                while (lower < upper) {
                    int middle = (lower + upper) >>> 1;
                    int c = comparator.compare(list.get(middle), key);
                    if (c < 0) {
                        lower = middle + 1;
                    } else { // c == 0
                        upper = middle;
                    }
                }
                return lower;
            }
        },
        /**
         * Return the index of the first list element that compares as greater than the key, or
         * {@code
         * list.size()} if there is no such element.
         */
        FIRST_AFTER {
            @Override
            public <E> int resultIndex(Comparator<? super E> comparator, E key, List<? extends E> list,
                    int foundIndex) {
                return LAST_PRESENT.resultIndex(comparator, key, list, foundIndex) + 1;
            }
        },
        /**
         * Return the index of the last list element that compares as less than the key, or
         * {@code -1} if there is no such element.
         */
        LAST_BEFORE {
            @Override
            public <E> int resultIndex(Comparator<? super E> comparator, E key, List<? extends E> list,
                    int foundIndex) {
                return FIRST_PRESENT.resultIndex(comparator, key, list, foundIndex) - 1;
            }
        };

        abstract <E> int resultIndex(Comparator<? super E> comparator, E key, List<? extends E> list, int foundIndex);
    }

    /**
     * A specification for which index to return if the list contains no elements that compare as
     * equal to the key.
     */
    public enum KeyAbsentBehavior {
        /**
         * Return the index of the next lower element in the list, or {@code -1} if there is no such
         * element.
         */
        NEXT_LOWER {
            @Override
            int resultIndex(int higherIndex) {
                return higherIndex - 1;
            }
        },
        /**
         * Return the index of the next higher element in the list, or {@code list.size()} if there
         * is no such element.
         */
        NEXT_HIGHER {
            @Override
            public int resultIndex(int higherIndex) {
                return higherIndex;
            }
        },
        /**
         * Return {@code ~insertionIndex}, where {@code insertionIndex} is defined as the point at
         * which the key would be inserted into the list: the index of the next higher element in
         * the list, or {@code list.size()} if there is no such element.
         *
         * <p>
         * Note that the return value will be {@code >= 0} if and only if there is an element of the
         * list that compares as equal to the key.
         *
         * <p>
         * This is equivalent to the behavior of
         * {@link java.util.Collections#binarySearch(List, Object)} when the key isn't present,
         * since {@code ~insertionIndex} is equal to {@code -1 - insertionIndex}.
         */
        INVERTED_INSERTION_INDEX {
            @Override
            public int resultIndex(int higherIndex) {
                return ~higherIndex;
            }
        };

        abstract int resultIndex(int higherIndex);
    }

    /**
     * Searches the specified naturally ordered list for the specified object using the binary
     * search algorithm.
     *
     * <p>
     * Equivalent to
     * {@link #binarySearch(List, Function, Object, Comparator, KeyPresentBehavior, KeyAbsentBehavior)}
     * using {@link Ordering#natural}.
     */
    public static <E extends Comparable> int binarySearch(List<? extends E> list, E e,
            KeyPresentBehavior presentBehavior, KeyAbsentBehavior absentBehavior) {
        checkNotNull(e);
        return binarySearch(list, e, Ordering.natural(), presentBehavior, absentBehavior);
    }

    /**
     * Binary searches the list for the specified key, using the specified key function.
     *
     * <p>
     * Equivalent to
     * {@link #binarySearch(List, Function, Object, Comparator, KeyPresentBehavior, KeyAbsentBehavior)}
     * using {@link Ordering#natural}.
     */
    public static <E, K extends Comparable> int binarySearch(List<E> list, Function<? super E, K> keyFunction,
            @Nullable K key, KeyPresentBehavior presentBehavior, KeyAbsentBehavior absentBehavior) {
        return binarySearch(list, keyFunction, key, Ordering.natural(), presentBehavior, absentBehavior);
    }

    /**
     * Binary searches the list for the specified key, using the specified key function.
     *
     * <p>
     * Equivalent to
     * {@link #binarySearch(List, Object, Comparator, KeyPresentBehavior, KeyAbsentBehavior)} using
     * {@link Lists#transform(List, Function) Lists.transform(list, keyFunction)}.
     */
    public static <E, K> int binarySearch(List<E> list, Function<? super E, K> keyFunction, @Nullable K key,
            Comparator<? super K> keyComparator, KeyPresentBehavior presentBehavior, KeyAbsentBehavior absentBehavior) {
        return binarySearch(Lists.transform(list, keyFunction), key, keyComparator, presentBehavior, absentBehavior);
    }

    /**
     * Searches the specified list for the specified object using the binary search algorithm. The
     * list must be sorted into ascending order according to the specified comparator (as by the
     * {@link Collections#sort(List, Comparator) Collections.sort(List, Comparator)} method), prior
     * to making this call. If it is not sorted, the results are undefined.
     *
     * <p>
     * If there are elements in the list which compare as equal to the key, the choice of
     * {@link KeyPresentBehavior} decides which index is returned. If no elements compare as equal
     * to the key, the choice of {@link KeyAbsentBehavior} decides which index is returned.
     *
     * <p>
     * This method runs in log(n) time on random-access lists, which offer near-constant-time access
     * to each list element.
     *
     * @param list the list to be searched.
     * @param key the value to be searched for.
     * @param comparator the comparator by which the list is ordered.
     * @param presentBehavior the specification for what to do if at least one element of the list
     *        compares as equal to the key.
     * @param absentBehavior the specification for what to do if no elements of the list compare as
     *        equal to the key.
     * @return the index determined by the {@code KeyPresentBehavior}, if the key is in the list;
     *         otherwise the index determined by the {@code KeyAbsentBehavior}.
     */
    public static <E> int binarySearch(List<? extends E> list, @Nullable E key, Comparator<? super E> comparator,
            KeyPresentBehavior presentBehavior, KeyAbsentBehavior absentBehavior) {
        checkNotNull(comparator);
        checkNotNull(list);
        checkNotNull(presentBehavior);
        checkNotNull(absentBehavior);
        if (!(list instanceof RandomAccess)) {
            list = Lists.newArrayList(list);
        }
        // TODO(lowasser): benchmark when it's best to do a linear search

        int lower = 0;
        int upper = list.size() - 1;

        while (lower <= upper) {
            int middle = (lower + upper) >>> 1;
            int c = comparator.compare(key, list.get(middle));
            if (c < 0) {
                upper = middle - 1;
            } else if (c > 0) {
                lower = middle + 1;
            } else {
                return lower
                        + presentBehavior.resultIndex(comparator, key, list.subList(lower, upper + 1), middle - lower);
            }
        }
        return absentBehavior.resultIndex(lower);
    }
}
